The invention concerns the design of an improved brush seal. In particular, the invention relates to a brush seal in which the bristle pack is, effectively, sub-divided into upstream and downstream portions by at least one annular diaphragm.
The basic configuration of a brush seal comprising a simple bristle layer, even with a deep backing plate, does not cope well with high pressure differentials and in regions of disturbed swirling airflow on the high pressure side, for example. A number of factors seem to contribute to poor seal performance in these conditions. On one hand, conditions of high pressure difference causes the bristles to become locked together so that they behave more like a solid mass. When this occurs the bristles are slow to track relative lateral movements of the component against which the seal is produced, for example a rotating shaft, so that once pushed aside by a transient movement the bristles lock-up leaving an increased leakage gap. If the bristles become locked in their undeflected positions initial leakage is low but the tips of the bristles rapidly wear against the co-operating seal surface on the shaft, and the wear rate is increased by shaft excursions, irregularities etc. On the other hand, in disturbed conditions the bristles are caused to flutter with detrimental effect on their co-operative ability to seal effectively because the bristles become permanently splayed destroying their seal properties. Furthermore the deterioration begins with the bristles at the exposed surface and progresses deeper into the layer. Initially the seal appears to perform as expected because the sealing function falls upon bristles further into the layer until eventually all the bristles are corrupted. These two effects may be combined so that while exposed bristles are disturbed and becoming permanently splayed the burden of sealing a high pressure differential falls on bristles towards the back of the layer which are locked together and subject to rapid wear.
Previous attempts to solve these problems, but without long lasting success, have involved assemblies of multiple brush seals. However, it has been found that under dynamic conditions several stages of adjacent brush seals do not contribute equally and a major proportion of the pressure drop across the whole seal assembly is borne by a single stage. Where turbulent conditions prevail the first, upstream seal has a calming influence on the leakage flow and the second or final stage bears most of the pressure drop. The said one stage gives about equal performance to a single stage seal and overall almost nothing is gained in the long term.